Air bearings for vibration isolation are known. For example, European patent application EP 1 803 965 entitled “Isolator Geometry of a vibration isolation system” (application number 06 026 425.1) shows an air bearing with increased bearing force. The disclosure of this document is incorporated herein by reference.
Air bearings of this type have a mechanical coupling element between a vertical piston and a piston plate or a load, respectively. In this way, while vertical forces are transferred to the piston plate, horizontal forces are essentially not. Also, changes of the parallelism of the piston plate with respect to the base plate are not transferred, which prevents tilting of the isolator pistons within each other.
Without taking into consideration isolator-internal structural resonances, known vibration isolation systems according to the air bearing principle are designed for a frequency range up to approximately 100 Hz (max. 300 Hz). However, with the continuing enhancement of resolution and precision of lithography devices, steppers and immersion systems, frequency characteristics above 300 Hz become more and more important. Therefore, it is important to optimize the frequency behaviour of the air bearing itself to maintain the isolation effect even when exceeding 300 Hz. Besides other effects that are relevant in prior art air bearings, the mechanical coupling element is of decisive importance. On the one hand, the weight to be supported bears thereon, on the other hand, it is the only element between the load to be supported and the isolator. The element which is screwed in the piston plate (at its upper end) is neglectable in comparison to the lower element which is screwed in the vertical piston of the air bearing, because it is considerably smaller. The lower element (which is referred to as pin) exhibits a mechanical stiffness which depends on the employed material, the length of the pin, the thread thickness of the screw connection and the supporting area of the screw connection. In known air bearings, this mechanical stiffness is adapted to be sufficient in case of a static load, but is particularly not designed as a dynamic stiffness and for dynamic resonances.
In known air bearings, a deflection of the pin is likely to be caused which together with the piston and the air cushion to the horizontal bearing, forms a vibratory system, with eigenfrequencies mostly above 300 Hz. As a result, the isolation effect is reduced in frequency ranges above 300 Hz.
It has been realized that this effect may only be reduced partially by changing the system parameters “bearing load” and “air cushion to the horizontal bearing”.